1. Technical Field
The present invention relates generally to semiconductor power devices, and more specifically to a circuit for protecting against an increase in output current for an integrated circuit power device driving an inductive load.
2. Description of the Prior Art
In electronic ignition systems for internal combustion engines, an ignition coil constitutes an inductive load. A semiconductor power switching device, normally a transistor, is used to drive the inductive load. The switching device is controlled using input voltage impulses having a duration such as to allow the current passing through the coil to reach a predetermined value.
The switch's input voltage can vary between a low value and a high value. In a typical application, a low input voltage causes the switch to be open, and a high input voltage closes the switch, thus allowing current to pass through the coil. If the switch is a bipolar transistor, so long as the input voltage is low the transistor's collector current is substantially zero and the transistor is off. In this event, the transistor behaves like an open switch. When the input voltage goes high, the transistor is on and behaves like a closed switch. As the input voltage goes high, the transistor's collector current, which is the power device's output current, starts to rise in a linear manner.
If, for accidental reasons, the input voltage continues to remain high beyond the expected time, the transistor remains on. Since the semiconductor switch continues to thus remain closed, the collector current consequently continues to rise. In this case, the collector current is limited only by the series resistance of the coil's primarily winding and of the switch. Such an enormous increase in the collector current causes an unacceptable current dissipation in the transistor, typically destroying it.
The transistor can also be destroyed by making use of the traditional method used in electronic ignition systems of limiting the collector current to a maximum preset value. In this case, the transistor falls out of saturation, thereby operating in the linear zone, and consequently almost all of the coil's supply voltage falls across the transistor itself. This causes a power dissipation that can destroy the transistor in only a few seconds.
A known method for protecting the power transistor is that is switching it off when the collector current reaches a preset value. This switching is performed independently of the fact that the input control voltage continues to remain high for an indefinite time. In such case, the protection circuit provides a clamping, or latch, circuit which uses a comparator to sense that the power transistor's collector current has reached a maximum value. The protection circuit then places itself in a state wherein it operates a control circuit of the power transistor, causing the transistor itself to be switched off.
The clamping circuit then remains in this state, ignoring other inputs that might reach it from the comparator, until it is reset to its primary state by a signal obtained from the input voltage when this goes low.
When this protection circuit is fabricated in the form of an integrated circuit on the same silicon chip as the power transistor and the protection circuit, considerable operating problems can be exhibited. An example of a technology which allows such construction is a vertical flow structure known as VIPower, available in products manufactured by SGS-Thomson Microelectronics. During switching of the power transistor, or Darlington if such is provided, the collector of the transistor itself has its voltage potential driven to a few volts below ground. This leads, consequently, to the switching on of vertical parasitic transistors inherent in the technological structure of the integrated protection circuit.
Obviously, the switching on of the parasitic transistor jeopardizes the correct operation of the protection circuit if it is indeed formed using an integrated technology as described above. This occurs because there is a clamping, or latch, circuit within the protection circuit, and switching on the parasitic transistors can, in fact, cause the erasure of the information stored in the clamping circuit itself. As a consequence, the protection circuit described above does not solve the problem of the destruction of the power transistor when the circuit itself is fabricated in an integrated form with the power transistor.
To understand this concept better, it is useful to consider the following typical example. As an initial condition, the input voltage is low and the power transistor is off. The voltage across the power transistor's collector is equal to the supply voltage, and the voltage applied across the ignition coil is zero. No current flows through the coil primary.
When the input voltage goes high, the power transistor is switched on and the voltage across its collector is equal to the saturation voltage. After a first interval of perhaps some 50 microseconds, depending on the particular features of the ignition coil, there is a first voltage undershoot (i.e., a voltage transient below ground) that lasts approximately ten microseconds. After this time, the collector voltage of the power transistor again becomes positive and equal to the saturation voltage. When the collector current reaches a preset maximum value, the clamping circuit intervenes and switches off the power transistor's control circuit. This causes the collector current to be cut off, and an overvoltage is created across the power transistor's collector that causes a spark on the coil's secondary. During the discharge of the secondary coil, the collector voltage maintains a value that is intermediate between the supply voltage and ground. When the discharge is over, which typically occurs after a second interval approximately two milliseconds from the time the collector current is switched off, there is a second collector voltage undershoot. In a typical case, this second undershoot lasts approximately ten microseconds.
If the protection circuit is fabricated in a single integrated device with the power transistor, the following problems occur during operation of the protection circuit. After the first time interval the clamping circuit must be inactive. However, the first undershoot of the power transistor's collector voltage can activate the clamping circuit and undesirably switch off the power transistor's control circuit. After the second time interval, the clamping circuit must be active even if the input voltage continues to remain high. Since a voltage undershoot occurs, the clamping circuit can lose the information that it had stored, and thereby reset itself. This would result in again switching on the control circuit.
It is therefore an object of the present invention to provide a protection circuit which can be fabricated in an integrated form with a power device. It is another object to provide such a circuit while avoiding the drawbacks mentioned above.